Waves in a periodic medium undergo coherent interference by multiple scattering when their wavelengths are comparable to the period spacing of the medium. Often, this results in the generation of distinct directions for waves having a certain energy to propagate. Therefore, an array comparable in period to the wavelength of electromagnetic waves can provide an analog i.e., a “bandgap,” which can act as a filter for a particular wavelength. Such arrays are the focus of international research and development efforts due to the tremendously broad range of applicability of optical systems.
A crystalline colloidal array (CCA) is one example of such a periodic medium. A CCA comprises a three-dimensionally ordered lattice that can be composed of almost any self-assembled monodisperse colloidal particles. These ordered systems can be fabricated to diffract electromagnetic radiation, including the visible spectrum. The diffraction characteristics of CCA systems is most accurately predicted through the application of dynamic diffraction theory, though Bragg's law is a reasonable approximation. Under certain conditions, these CCA's can exhibit a photonic bandgap over a narrow range of the spectrum. Such arrays hold promise as a practical route to generating optical photonic crystals, which may be employed in many optical applications, including active photonic switching and sensory applications.
CCA's are the focus of ongoing research and development. For example, a pending provisional application that is owned by the Assignee of the present application is U.S. Provisional Application No. 60/327074 filed Oct. 3, 2001. This application is directed to a tunable radiation filter which includes a highly ordered crystalline array of particles fixed in an essentially water-free matrix.
In the past, these materials have been developed in a liquid phase. Unfortunately, liquid phase CCA materials do not exhibit a high level of robustness or stability. For instance, a liquid phase CCA will undergo a transitory disordering when subjected to a mechanical shock, while a permanent disordering can be induced to occur with the introduction of ionic impurities.
Prior art publications describe methods of making solid filter materials that filter a predetermined wavelength band. For instance, U.S. Pat. No. 6,001,251 to Asher et al. discloses creating a colloidal structure composed of particles dispersed within a medium, and introducing a solvent thereto. The solvent is then evaporated and the remaining structure crystallizes.
Other prior art publications have been directed to methods for “tuning” CCA's to particular band gaps for specific filtration or sensory applications. For example, U.S. Pat. No. 6,014,246 to Asher et al. is directed to mesoscopically periodic materials that combine CCA self-assembly with the temperature induced volume phase transitions of various materials.
Due in part to problems in the past concerning the robustness of liquid-phase CCA's, these systems have had limited practical applicability. Attempts have been made to stabilize CCA's using various materials, but again, the systems have had limited practical applicability due to, for example, the nature of the stabilizing material. For example, various acrylamides have been used in the past to aid in stabilizing a CCA. Such materials, however, are ineffective in certain applications, such as certain biologically based applications. Thus, many prior art products and methods are inapplicable for a host of biologically based applications, including, for example, sensory applications involving recognition of various microorganisms.
As a result, a composition comprising a CCA which provides good mechanical and optical properties would be very desirable. Additionally, it would be desirable to develop a CCA system which can provide not only the robustness necessary for practical applications, but also be useful in biological applications.